The present invention relates to refractories suitable for use as auxiliary tools to support the work being fired to fabricate a variety of ceramic products including ceramic electronics parts (e.g. ceramic capacitors, piezoelectric devices and IC packages) and ceramic sliding materials.
Refractories for use as auxiliary tools in the process of firing ceramics are required to have heat resistance and mechanical strength that are appropriate for specific conditions of use. On the other hand, such refractories should be heat-resistant at the lightest weight in order to reduce the thermal energy they consume in the furnace and to shorten the durations of time required for temperature elevation and cooling, thereby reducing the energy cost while increasing the production rate. Besides these requirements, there is a characteristic need for the absence of reactivity with the work or ceramics that are to be fired. If the auxiliary tool or support reacts with the work at the interface, disadvantages such as fusion and deterioration of performance due to compositional changes will occur. In order to avoid this problem, it is desired that the auxiliary tool does not enter into any kind of reaction with the work under the firing conditions being employed. It is also desired that the surface of the auxiliary tool is as smooth as possible in order to insure that the surface properties of the work will not be damaged by contact with the support. These requirements have become increasingly stringent for auxiliary tools that are employed in the firing of modern miniature precision-molded electronics parts.
Refractories fabricated by shaping mixtures of refractory particles and heat-resistant inorganic fibers using inorganic binders (as described in JP-A-63-206367 and 59-88378 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") are lightweight, exhibit high heat resistance and have sufficient strength to support lightweight work such as electronics parts. However, these refractories are somewhat poor in surface properties such as smoothness and reactivity. Furthermore, they are porous and gas-permeable, so that they tend to deteriorate if hot corrosive gases get into their bulk during use.
Improvements on the surface properties of lightweight refractories for use as auxiliary tools in firing ceramics are described in JP-A 62-216974 and 62-283885. According to their teachings, a zirconia or alumina coating is applied onto refractories to improve their surface properties.